The present invention relates to the identification of compounds that act, at physiologically-compatible levels, to inhibit the formation of proteinaceous tissue deposits denoted generally as "amyloid". More specifically, the present invention relates to pharmaceutically active agents that impede formation of amyloid fibrils in vivo, and to a method for the screening of compounds which posses this activity.
Classified under the rubric "amyloidosis" are a number of pathological conditions characterized by the deposition of abnormal fibrils ("amyloid fibrils") in extracellular spaces. The amyloid fibril, in turn, represents a final common pathway for a diverse array of proteins. Regardless of their biochemical composition, however, all types of amyloid fibrils share (a) a .beta.-pleated sheet structure, (b) green birefringence under polarized light after staining with Congo Red dye, and (c) a fibrillar morphology which has a typical electronmicroscopic appearance.
The deposition of amyloid fibrils can affect several organs in the systemic forms of the disorder, exemplified by familial Mediterranean fever, familial amyloid polyneuropathy and systemic amyloidosis, or it can be restricted to one organ in localized forms. Among the latter are conditions classified under the rubric "cerebral amyloidosis," which covers the Alzheimer group of diseases, namely, Alzheimer's disease [pre-senile dementia, senile demential]; Alzheimer's disease associated with Down's syndrome; Alzheimer's disease associated with other central-nervous-system diseases, such as Parkinson's disorder; and congophilic angiopathy [associated or not associated with Alzheimer's disease].
There is no effective therapy for cerebral amyloidosis, which almost invariably has a fatal outcome following the onset of amyloid deposits. For example, Alzheimer's disease is estimated to be the fourth or fifth leading cause of death among North Americans.
A universally accepted indicator of cerebral amyloidosis is the accumulation of large numbers of lesions, so-called "senile plaques", that are comprised in large part of amyloid fibrils. Senile plaques are spherical, ranging from 10 to 200 .mu.m in diameter, and are found occasionally in aged adult cerebral cortex but in large numbers in Alzheimer-affected cortex.
The utilizing of materials found in human brain (normal of Alzheimer-affected) that are not already amyloid, and of transforming them into amyloid, has not been documented in the literature. There was also no description in the art of an experimental system, derived exclusively from human materials, that was characterized by the feature of Alzheimer's disease. Because the presence of amyloid is the most qualitatively and quantitatively specific indication of senile-plaque formation, most specialists agree that reproduction of amyloid fibrils experimentally from precursor materials which are extracted, activated, or otherwise derived from human brain would constitute the best available evidence linking an agent or precursor to the progression of cerebral amyloidosis.
Despite the recognized importance of an experimental system that would permit testing for such a linkage, it has been possible to reproduce amyloid experimentally from materials derived solely from human brain tissue. Accordingly, there has been no reliable indicator available for compounds that might be effective in treating cerebral amyloidosis; nor has it been possible to determine whether a group of compounds exists that block the conversion of a brain-localized precursor to cerebral amyloid (i.e., that display "anti-amyloid activity") at physiologically acceptable levels of the active agent.
A microscopic structure referred to as the dense microsphere is known to exist in normal brain and in brain affected by Alzheimer's disease. See Averback, Acta Neuropathol. 61: 148-52 (1983); results confirmed by Hara, J. Neuropath. Exp. Neurol. (1986). Evidence for the existence of DMS comes from microscopic histological section studies of fixed whole brain tissue, where the dense microspheres are seen to have a proteinaceous central region ("DMS protein") surround by continuous membrane ("DMS membrane"). The dense microspheres are observed as randomly dispersed, very infrequent structures which occupy an estimated 10.sup.-9 or leas or total brain volume, at a unit frequency roughly estimated at 10.sup.-16 or less, relative to other definable brain structure such as mitochondria.
Neither the extraction, purification and characterization of isolated samples of DMS nor the use of DMS material to any advantage has been documented. Thus, DMS are structures of unproven function and unknown significance or usefulness, and have been effectively unavailable in tangible form.